Carbon nanotubes have anisotropic structures with a variety of shapes, including single-walled, multi-walled, and bundled into rope-like multi-tube structures, among others. Carbon nanotubes typically range in diameter from about fractions of a nanometers to several tens of nanometers, and range in length from about several microns to several millimeters. Carbon nanotubes also exhibit conductive or semiconductive properties depending on their chirality. For example, it is generally recognized that carbon nanotubes having an arm-chair structure exhibit metallic properties, whereas carbon nanotubes having a zig-zag structure exhibit semiconductive or metallic properties depending on diameter.
In addition, it has been observed that the electronic and, perhaps, the mechanical characteristics of carbon nanotubes, such as single wall carbon nanotubes, may be governed by their chirality, and that their chirality may in turn be governed by the diameter of the catalysts from which the nanotubes are grown (Nasibulin et. al., Carbon 43 (2005), 2251-2257). Chirality often refers to the roll-up vector for the nanotube. Chirality has been described extensively in the literature (Satto et al., Physical Properties of Carbon Nanotubes, Imperial College Press (2004) pg 37), and may be specified by a vector Ch represented as:Ch=ma1+na2 where a1 and a2 are real vectors of a hexagonal sublattice of graphite constituting the surface of the carbon nanotube.
In such a vector, when n-m is divisible by 3, the carbon nanotube is believed to exhibit metallic properties. Otherwise, the carbon nanotube is believed to exhibit semiconductive properties. For those carbon nanotubes exhibiting semiconductive properties, their band gap may also be affected by and change with the chiral vector. For certain applications, such as photonic detectors, or for transistor synthesis, control of the chiral vector (i.e., chirality) can be critical. Given that the diameter of carbon nanotubes can be expressed asdt=a[n2+m2+nm]1/2,it should be appreciated that with a very small change in the nanotube diameter, i.e., dt, there can be a significant effect on electronic character of the nanotube.
As an example, if “m” and “n” determine the metallic or semiconductor characteristic of the carbon nanotube, then changes in the electronic character of the carbon nanotube can occur with changes in its diameter “d”. The sensitivity, at the level of an individual carbon nanotube, can be in fractions of a nanometer. As a result, chirality control through control of the diameter, at present, can be difficult, if not impossible. For instance, if n=6 and m=5, then by definition d=0.948 a in nanometers (nm). However, if “n” were maintained such that n=6 and “m” were changed so that m=3, then d=0.793 a nm. The latter nanotube, with a slight change in “m”, becomes a metallic conductor, whereas the former is a semiconductor.
It is well accepted that applications using carbon nanotubes can be wide-ranging, including those in connection with memory devices, electron amplifiers, gas sensors, microwave shields, electrodes, electrochemical storage, field emission displays, and polymer composites among others. Specifically, semiconducting carbon nanotubes can be used, for instance, in memory devices, sensors, etc., while metallic carbon nanotubes may be used in electrode materials of cells, electromagnetic shields, etc. To make these applications practical using carbon nanotubes, it will, therefore, be necessary to obtain and/or created carbon nanotubes with a specific diameter or diameter range, in order to obtain carbon nanotubes with a specific chirality.
Selection between the metallic and semiconductive characteristics, therefore, requires a substantially precise ability control of the catalyst diameter. In some instances, the accuracy needs to be better than about 0.155 nm. However, it should be noted that this difference can become even closer, as “m” and “n” become large. Adding to the difficulty is the ability to precisely control the catalyst diameter during the growth process. In particular, if the catalysts are in a molten state (Applied Physics Letters 87, 051919 —2005_), the presence of droplet vibrations can likely introduce considerable diameter variations in the resulting carbon nanotube generated. If, on the other hand, the catalysts are in a crystalline state, these catalysts are likely formed from metallic clusters that also vary in diameter. As a result, carbon nanotubes generated from such metallic clusters can also vary in diameter.
There exist several historic approaches that have been taken to select, for example, single wall carbon nanotubes of a given chirality. These include: (1) attempts to control diameter of the catalyst particle (Katauraa et al., Diameter Control of Single-walled Carbon Nanotubes, Carbon 38 (2000) 1691-1697), (2) epitaxial growth of nanotubes on fragments of known chirality (U.S. Pat. No. 7,052,668), (3) using electric discharging or laser deposition to produce nanotubes having specific chirality, and (4) selection of only those tubes meeting the desired chirality after a batch of tubes have bee made and processed (Feringa et al. Molecular Chirality Control and Amplification by CPL: Correction, Science 276 (5311) 337-342). Of these, the last one seems to offer the most promise. However, it has been observed that such an approach can be destructive, may not let an operator preselect chirality with great accuracy (http://www.fy.chalmers.se/conferences/nt05/abstracts/P357), and can also be time consuming.
Accordingly, it would be desirable to provide an approach that can permit a predetermined chirality to be specified or defined substantially precisely, so that nanotubes with such specified chirality can subsequently be fabricated, and which approach can permit a volume of substantially uniform nanotubes with substantially uniform chirality to be obtained.